Systems And Methods For Scaling A Signal In A Power Factor Correction Circuit

ABSTRACT

Systems and methods for scaling a current signal in a power factor correction circuit are disclosed. An exemplary method may include providing a power factor correction circuit for a power supply, the power factor correction circuit having a first current sensing resistor connected on a return path to a rectified AC line. The method may also include measuring current across the first current sensing resistor. The method may also include switching on at least a second current sensing resistor in parallel with the first current sensing resistor if the measured current increases above a threshold value.

BACKGROUND

The power factor (PF) of an alternating current (AC) electric circuit isthe ratio of real power to apparent power, and is expressed as a numberbetween 0 and 1.0 (or as a percentage). Real power is the capacity ofthe circuit to perform work in a given time, and apparent power is theproduct of the current and voltage of the circuit. Various factors(e.g., a non-linear load, or the amount of energy stored in the loadversus energy returned to the power source) can cause the apparent powerto exceed the real power, increasing power losses through the utilitycompany's electrical transmission and distribution lines. Utilitycompanies may even charge higher rates to customers who do not maintainhigh power factors.

Accordingly, it is often desirable to adjust the power factor of anelectronics system (e.g., a computer server or collection of serverssuch as a “server farm”). Power factor correction (PFC) circuits areavailable that bring the power factor of an AC circuit closer to 1.0.Typical PFC circuits operate by determining the PF and adding capacitorsand/or inductors to cancel the inductive or capacitive effects of theload. The PF can be determined by dividing the power (in Watts) by theproduct of measured voltage and current. Therefore, it is important forthe voltage and current measurements to be accurate.

Sensing elements for measuring voltage and current in PFC circuits aregenerally sized to minimize power loss for the highest rated outputpower at the lowest rated input voltage. But when operating at thelowest rated output power at the highest rated input voltage, thevoltage signal generated by the sensing element can be very small, witha poor signal to noise ratio. The decreased accuracy in this measurementmakes it more difficult to maintain an optimal PF.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level illustration of an exemplary computer system andthe resulting PF under load, wherein (a) shows a plot with a laggingpower factor (PF<1.0), (b) shows a plot with a PF at or near 1.0.

FIG. 2 is a schematic diagram illustrating an exemplary PFC circuitwhich may be implemented for scaling a signal.

FIG. 3 is a flowchart illustrating exemplary operations for scaling asignal in a PFC circuit.

DETAILED DESCRIPTION

Systems and methods described herein may be implemented in a powerfactor correction (PFC) circuit for scaling a signal (e.g., a currentsignal). The PFC circuit may be provided for a power supply in anelectronics systems (e.g., one or more computer server). In an exemplaryembodiment, the PFC circuit may include a first current sensing resistorconnected on a return path to a rectified AC line for a currentmeasurement. The current sensing resistor has to be sized for a maximumexpected load. For example, the resistor may be sized for a 10 Amp loadfrom the AC outlet. However, if the electronics system is drawing lesscurrent (e.g., 1 Amp), the current signal is much smaller and lessaccurate. Accordingly, current across the first current sensing resistormay be measured, and at least a second current sensing resistor may beswitched on in parallel with the first current sensing resistor if themeasured current increases above a threshold value. Likewise, at leastthe second current sensing resistor is switched off if the measuredcurrent decreases below the threshold value. Of course, multipleresistor/switch elements may be added to scale the signal across anydesired range of the PFC circuit.

According to exemplary embodiments described herein, the signalamplitude may be increased without increasing power losses across a fullrange of operation of the electronics device. The increased signal levelresults in improved input measurement accuracy, which may be used toincrease the power factor (PF) across a full operating range of the PFCcircuit.

Exemplary System

FIG. 1 is a high-level illustration of an exemplary computer system andthe resulting PF under load, wherein (a) shows a plot 100 with a laggingpower factor (PF<1.0), (b) shows a plot 101 with a PF at or near 1.0.The example shown in FIG. 1 a corresponds to an electronics system(e.g., one or more server computers) without any PF correction. It canbe seen from the example shown in plot 100 that the current waveform 110lags the voltage waveform 120 by about 75 degrees (note the lagillustrated by bracket 105). As discussed above, operating under theseconditions is undesirable for a number of reasons.

A PFC circuit (e.g., the PFC circuit 200 in FIG. 2) may be implementedin the electronics system, e.g., as part of the power supplyelectronics. The PFC circuit 200 functions to correct lag between thecurrent signal 110 and the voltage signal 120 in order to approach ormeet a PF of 1.0, as illustrated by the plot 101 in FIG. 1 b. It isobserved in the plot 101 that after correction the lag is much less(indeed, in FIG. 1 b there is no lag as illustrated by reference 115)between current signal 111 and voltage signal 121.

The PFC circuit 200 may correct lag by determining the PF and addingcapacitors and/or inductors to cancel the inductive or capacitiveeffects of the load. The PF can be determined based on voltage and/orcurrent measurements. Therefore, it is important for these measurementsto be accurate. However, when the electronics system is drawing lesscurrent than the sensing element is sized for (e.g., drawing 1 Ampinstead of 10 Amps), the signal may be too small for accuratemeasurements. Accordingly, the signal may need to be amplified orscaled.

FIG. 2 is a schematic diagram illustrating an exemplary PFC circuit 200which may be implemented for scaling a signal. The PFC circuit 200 mayinclude a plurality of current sensing resistors 210 a, 210 b connectedon a return path 202 of a rectified AC line. The plurality of currentsensing resistors 210 a, 210 b may be used for the current measurement.

In an exemplary embodiment, the number of current sensing resistors 210a, 210 b which are switched in may be varied to scale the current signalbased on the operating conditions. For example, the first resistor 210 amay be sized for the lowest expected current (e.g., 1 Amp) to provide alarger signal sense. If the electronics system is drawing more currentthan the first current sensing resistor 210 a is sized for, the currentsignal may not be suitable for accurate measurements. Accordingly, oneor more additional current sensing resistors 210 b may be switched on inparallel with the first current sensing resistor 210 a when the measuredcurrent increases above a threshold value, to lower the equivalentresistance while maintaining the signal level. Likewise, the additionalcurrent sensing resistors 210 b may be switched off if the measuredcurrent decreases back below the threshold value.

It is noted that any suitable threshold (or thresholds) may beimplemented and may depend on various design considerations. Forexample, multiple threshold intervals may be implemented to switchon/off current sensing resistors for different intervals. Exemplarydesign considerations may include, but are not limited to, sizing of thecurrent sensing resistors, the number of current sensing resistors, thedesired granularity, the desired ability for scaling, and the desiredlevel of signal magnification.

Exemplary PFC circuit 200 is shown in FIG. 2 as it may be implemented inhard-wired circuitry. However, it is noted that the circuit may also beimplemented in other circuitry (e.g., logic gates) as will be readilyapparent to those having ordinary skill in the art after becomingfamiliar with the teachings shown and described herein.

The PFC circuit 200 may be connected on lines 201, 202 between a bridge(not shown) to an AC power source 205 (e.g., an electrical outlet) and aload 215 (e.g., a server computer). The bridge provides a rectified ACsignal which behaves as a “partial DC” signal. Bridges for providing arectified AC signal are well known in the electronics arts, andgenerally operate by “flipping” the negative portion of the AC sine waveso that it is additive with the positive portion of the AC sin wave.Capacitor 220 serves as a high frequency filter element.

The PFC circuit 200 may include a boost circuit 230 to provide a powersupply “boost” to the load 215. An exemplary boost circuit 230 mayinclude an inductor 232 controlled by the field-effect transistor (FET)234 and diode 236. The boost circuit 230 boosts the voltage supplied onthe voltage bus (Vbus) 201. A capacitor 240 may be provided to hold thecharge. The return bus (Vrtn) 202 provides a path back to the AC powersource 205.

Boost circuits such as the one just described are well-understood in theelectronics arts, and the specific components called out above aremerely illustrative of one type of boost circuit which may beimplemented. Other types of boost circuits may also be used, as will bereadily understood by those having ordinary skill in the art afterbecoming familiar with the teachings herein.

A plurality of current sensing resistors 210 a, 210 b may be provided onthe return line 202. Although only two current sensing resistors 210 a,210 b are shown in FIG. 2, it is noted that any number of currentsensing resistors may be implemented.

The output from the current sensing resistors 210 a, 210 b may be usedfor the boost circuit 230 (e.g., as a current measurement 212) tocontrol output the desired level of PF correction via FET 234. The firstresistor 210 a is connected in-line on the return bus 202, and may beselected or sized for a minimum expected load (e.g., 1 Amp). The secondresistor 210 b is switchable in and out of the circuit using a switchingelement 250.

In operation, when the load increases above a threshold value (e.g.,over about 1 Amp), the second resistor 210 b combines with the firstresistor 210 a to provide an overall lower resistance on the return bus202 for accurate current measurements at higher currents (e.g., byamplifying the signal up to about 10 Amps).

When the load decreases (e.g., below 1 Amp), switching element 250 isactivated to switch off or “remove” the second resistor 210 b from thereturn bus 202. At which time, the first resistor 210 a provides anoverall higher resistance on the return bus 202 for accuratemeasurements at lower current.

In exemplary embodiments, the current-sensing switch may be selectedbased on threshold values for magnifying the amplitude of the currentsignal for accurate current measurements. Although any suitableswitching element may be implemented, in an exemplary embodiment theswitching element 250 may be a current-sensing switch whichautomatically turns on/off at a predetermined current level, henceconnecting/disconnecting the second resistor 210 b in-line on the returnbus 202.

It is noted that multiple switching elements (or a single switchingelement operable to switch in multiple resistors) may also beimplemented where more current intervals are desired, or for failoverpurposes.

Before continuing, it is noted that the PFC circuit 200 may beconfigured at run-time so that the current sensing path is configuredbased on operational data for the circuit. Also in exemplaryembodiments, configuration of the current sensing path may be maintainedduring operation. Accordingly, the current sensing path is adaptable andmay be reconfigured, e.g., based on changes in the run-time environment.

It is also noted that the systems and methods described herein do notneed to be implemented in any particular circuit design. The circuitdesign described with reference to FIG. 2 is provided merely asexemplary of one embodiment of a circuit. It is contemplated that thosehaving ordinary skill in the art, after becoming familiar with theteachings herein, will be able to provide other circuit designs forscaling a current signal in a power factor correction circuit.

Exemplary Operations

FIG. 3 is a flowchart illustrating exemplary operations 300 for scalinga signal in a power factor correction circuit. In an exemplaryembodiment, the components and connections depicted in the figures maybe used. It is also contemplated that in other embodiments, operationsshown and described herein may be implemented in other circuitry, logiccomponents, and/or control logic such as a processor or processingunits.

In operation 310, current may be measured across a first current sensingresistor. In operation 320, a determination is made whether the currentmeasured in operation 310 drops below a threshold value. If the measuredcurrent increases above the threshold value, at least a second currentsensing resistor may be switched on in parallel with the first currentsensing resistor in operation 330. The determination 320 may also berepeated until the measured current increases above the threshold value.

It is noted that additional current sensing resistors may also beswitched on, e.g., depending on the desired accuracy of the currentmeasurement. In an exemplary embodiment, determination 320 may berepeated in a step-wise manner in order to discern how many currentsensing resistors should be switched on.

In operation 340, another determination is made whether the currentmeasured in operation 310 decreases below a threshold value. Inoperation 350, the additional current sensing resistor(s) may beswitched off if the measured current decreases below the thresholdvalue. Again, the determination multiple current sensing resistors maybe switched off based on a comparison to multiple threshold values.

The operations shown and described herein are provided to illustrateexemplary implementations for scaling a current signal in a power factorcorrection circuit. Still other operations may also be implemented.

In addition to the specific embodiments explicitly set forth herein,other aspects will be apparent to those skilled in the art fromconsideration of the specification disclosed herein. It is intended thatthe specification and illustrated embodiments be considered as examplesonly, with a true scope and spirit of the following claims.

1. A method for scaling a signal in a power factor correction (PFC)circuit, comprising: providing the PFC circuit for a power supply, thePFC circuit having a first current sensing resistor connected on areturn path to a rectified AC line; measuring current across the firstcurrent sensing resistor; and switching off at least a second currentsensing resistor in parallel with the first current sensing resistor ifthe measured current drops below a threshold value.
 2. The method ofclaim 1 further comprising switching on the second current sensingresistor if the measured current increases above the threshold value. 3.The method of claim 1 wherein the second current sensing resistor has alower resistance rating than the first current sensing resistor.
 4. Themethod of claim 1 wherein the second current sensing resistor inparallel with the first current sensing resistor has a lower resistancerating than the first current sensing resistor alone.
 5. The method ofclaim 1 wherein the second current sensing resistor switched on inparallel with the first current sensing resistor increases measurementsensitivity of a current signal on the return path.
 6. The method ofclaim 1 further comprising increasing signal amplitude to enhancecurrent measurement accuracy for a low amplitude current signal on thereturn line.
 7. The method of claim 1 further comprising switching on atleast a third current sensing resistor in parallel with the first andsecond current sensing resistors to scale a current signal on the returnline across a range of current signals.
 8. A system for scaling acurrent signal in a power factor correction (PFC) circuit, comprising: afirst current sensing resistor connected on a return path in the PFCcircuit; at least a second current sensing resistor; and a switchoperable to connect at least the second current sensing resistor inparallel with the first current sensing resistor if the measured currentincreases above a threshold value.
 9. The system of claim 8, wherein theswitch is operable to switch on the second current sensing resistor ifthe measured current increases above the threshold value.
 10. The systemof claim 8, wherein the second current sensing resistor has a resistancerating lower than or equal to a resistance rating of the first currentsensing resistor.
 11. The system of claim 8, wherein the second currentsensing resistor has a lower resistance rating than the first currentsensing resistor alone.
 12. The system of claim 8, wherein the secondcurrent sensing resistor decreases power loss of a current signal on thereturn path.
 13. The system of claim 8, wherein the second currentsensing resistor decreases power loss.
 14. The system of claim 13,wherein the second current sensing resistor increases currentmeasurement accuracy for a high current signal on the return line. 15.The system of claim 13, wherein power loss is unaffected or decreasesacross a full range of operation for equivalent measurement accuracy.16. The system of claim 8, further comprising at least one more than atleast the second current sensing resistor switchable in parallel withthe first current sensing resistor to further scale a current signal onthe return line across a range of current signals.
 17. A system forscaling a current signal in a power factor correction circuit,comprising: first current sensing means connected on a return path; atleast second current sensing means provided in parallel with the firstcurrent sensing means; and means for switching on at least the secondcurrent sensing means if current measured across the first currentsensing means increases above a threshold value.
 18. The system of claim17 further comprising means for switching off the at least secondcurrent sensing means if the measured current decreases below thethreshold value.